The word "nanotechnology" describes a constellation of fields and theories. There is little agreement to date on how far the technology can advance. In large part, this is because there is little agreement about what nanotechnology is. In particular, the limits of advanced nanotechnology are still a matter of controversy. This paper provides a definition for a particular type of nanotechnological capability, "molecular manufacturing". This definition is intended to encompass and ground much of the speculation about the ultimate capabilities of nanotechnology, while also being applicable to more mundane and near-term manufacturing-related nanotechnologies. The definition distinguishes simple nanoscale technologies that do not develop manufacturing systems from the more advanced nanotechnologies that do.